1. Field of the Invention
The present invention relates to telephone paystations, or coin telephones, and more particularly to a shock preventing handset for use with a telephone paystation.
2. Background Art
A number of techniques have been employed in the past to prevent the transmission of an electrical shock from a telephone paystation to the user. Usually very expensive, these techniques have included the use of waterproof handsets, as well as handsets which electrically isolate the transmitter and/or receiver via a transformer. It has also been in the case of paystations to utilize relays to isolate the handset during the application from the telephone central office of high voltage collect or refund potentials. Handsets have also been isolated by utilizing capacitors to isolate the transmitter and/or receiver.
A background art search directed to the subject matter of this application and conducted in the U.S. Patent and Trademark Office disclosed the following U.S. Letters Patent:
______________________________________ 4,672,666 4,974,258 4,821,320 5,081,674 4,845,760 5,237,608 British Patent 425,186 ______________________________________
None of the patents discovered in the search disclosed means for preventing the transmission of high voltage electrical shock resulting from the application of collect or refund potential at the central office to the user. Of the discovered patents, two related to the protecting of internal equipment from electrostatic discharge as well as providing some protection to the user from the same electrostatic charge. These patents included U.S. Pat. No. 4,821,320 which issued to Andert et al on Apr. 11, 1989. This patent is drawn to a device for protecting both operator of the telephone and the electronic equipments included in their from high electrostatic discharge. Protection is achieved by providing a contact path which has substantial ohmic resistance and which terminates in the apparatus at a ground point carrying a low potential.
U.S. Pat. No. 5,237,608 was granted to Otto et al on Aug. 27, 1993. This patent also pertains to a system that prevents the risk of arc over due to electrostatic discharge in the region of the joining location of the bottom tub and upper housing portion of the telephone, thus protecting electrical components contained within the telephone handset. Protection is provided by connecting the upper housing part to the bottom part with edge region adapted to the outside contour of the bottom tub. Thus the edge region overlaps the bottom tub over a large area.